In a security system there are two goals to be aimed at: one is the certainty of detection of a threat to the security of whatever is protected by the system; the other is freedom from false alarms. Unfortunately these two aims tend to be mutually contradictory and any practical system is a compromise between them. It is an object of this invention to provide a security system for detecting attempts to make an entry through sheet glass which aims to give a high degree of protection against deliberate threats to security while maintaining a relative freedom from false alarms due to various causes which are explained more fully below.
Thus the invention is particularly concerned with the protection of premises in which there are windows, glass doors or the like through which entry may be forced into the premises. Entry through a window which entails breaking the glass can be readily detected since the breakage will set up vibrations in the glass and these vibrations can be detected. Mechanically operating vibration detectors have been proposed in the past. Alternatively the vibrations can be detected by a transducer and converted to an electrical signal monitored at some remote point. However, a monitoring arrangement which responded to all vibrations of the glass would render the system highly liable to false indications of entry. For example, it is found that vibration of the glass due to wind, traffic vibration or tapping of the glass would produce an alarm indication as well as an attempt to actually break the glass, or as is more likely, to cut the glass in order to remove a portion of it to gain entry.